A field-emission cathode is an electronic device that emits electrons when subjected to an electric field of sufficient strength. The electric field is created by applying a voltage between the cathode and an electrode, typically referred to as the anode or gate electrode, situated a short distance away from the cathode.
A field emitter typically contains a group, often a very large group, of individual electron-emissive elements distributed across a supporting structure. This configuration is commonly referred to as an area field emitter. A variety of different shapes have been investigated for the electron-emissive elements in area field emitters.
Spindt et al, U.S. Pat. No. 3,665,241, discloses an area field-emission structure in which the electron-emissive elements consist of solid cones or pyramids situated in an ordered array over a flat substrate electrode. Each cone or pyramid lies directly on the substrate electrode, or is situated on a pedestal that lies on the substrate. The field emitter can include a top electrode spaced apart from the substrate and having a like-ordered array of openings through which the electron-emissive elements are exposed.
In fabricating the electron-emissive elements in Spindt et al, a screen having an appropriate array of circular or square openings is placed above the substrate electrode. A deposition is then performed simultaneously from two sources. One of the sources consists of an emitter-forming metal, such as molybdenum, atoms of which are deposited in a direction perpendicular to the substrate electrode. The other source consists of a closure material, such as a molybdenum-alumina composite. Atoms of the closure material are caused to impinge on the screen at a small angle to the substrate. The closure material progressively closes the openings in the screen as the emitter-forming metal passes through the openings to create the electron-emissive elements. The result is that the emitter-forming metal is deposited in the shape of cones or pyramids depending on whether the screen openings are circular or square.
Yoshida et al, U.S. Pat. No. 5,164,632, discloses a gated field-emission structure in which solid elongated gold electron-emissive elements are situated in pores extending through an alumina layer. An address line lying under the alumina layer contacts the lower ends of the electron-emissive elements. Their upper ends are pointed. A gate electrode situated above the electron-emissive elements extends slightly into the pores.
To manufacture their field emitter, Yoshida et al anodically oxidize part of an aluminum plate to create a thin alumina layer having pores that extend nearly all the way through the alumina. An electrolytic technique is used to fill the pores with gold for the electron-emissive elements. The address line is formed over the filled pores along the alumina side of the structure after which the remaining aluminum and part of the adjoining alumina are removed along the opposite side of the structure to re-expose the gold in the pores. Part of the re-exposed gold is removed during an ion-milling process utilized to sharpen the electron-emissive elements. Gold is then evaporatively deposited onto the alumina and partly into the pores to form the gate electrode.
Greene et al, U.S. Pat. No. 5,150,192, discloses a gated field emitter in which hollow elongated electron-emissive elements extend through a thin electrically insulating substrate. The electron-emissive elements have pointed tips that protrude into cavities provided along the upper substrate surface below the gate electrode. A metal film lies along the lower substrate surface.
In fabricating their field emitter, Greene et al create openings partway through the substrate by etching through a mask formed on the bottom of the substrate. Metal is deposited along the walls of the openings and along the lower substrate surface. A portion of the thickness of the substrate is removed along the upper surface. The gate electrode is then formed by a deposition/planarization procedure. The cavities are provided along the upper substrate surface after which the hollow metal portions in the openings are sharpened to complete the electron-emissive elements.
A large-area field emitter for an application such as a flat-panel television screen where the diagonal screen dimension is 25 cm needs a relatively strong substrate for supporting the field-emission components extending across the large emitter area. The requisite substrate thickness is typically several hundred microns to 10 mm or more. Due to the ways in which Yoshida et al and Greene et al manufacture their field emitters, it would be quite difficult to attach those emitters to substrates of such thickness. Consequently, Yoshida et al and Greene et al are not suited for scaling up to large-area field-emission applications.
Borel et al, U.S. Pat. No. 4,940,916, discloses a gated area field emitter in which the electron-emissive elements consist of cones formed on a highly resistive layer that overlies a highly conductive layer situated on an electrically insulating supporting structure. For a thickness of 0.1-1 micron, the highly resistive layer has a resistivity of 10.sup.4 -10.sup.5 ohm-cm. The resistive layer limits the currents through the electron-emissive cones so as to protect the field emitter from breakdown and short circuits.
Busta, "Vacuum Microelectronics-1992," J. Micromech. Microeng., Vol. 2, 1992 pp. 43-74, provides a general review of field-emission devices. Among other things, Busta discusses Utsumi, "Keynote Address, Vacuum Microelectronics: What's New and Exciting," IEEE Trans. Elect. Dev., October 1990, pp. 2276-2283, who suggests that a filament with a rounded end is the best shape for a field-emission element. Busta also discusses Betsui, "Fabrication and Characteristics of Si Field Emitter Arrays," Tech Dig IVMC 91, 1991 pp. 26-29, who utilizes a lift-off technique in forming a field-emitter array. Also of interest is Fischer et al, "Production and use of nuclear tracks: imprinting structure on solids," Rev. Mod. Phys., October 1983, pp. 907-948, which deals with the use of charged-particle tracks in manufacturing field emitters according to a replica technique.